The invention relates to an assembly for shutting off a pipe mouth, comprising an essentially tubular mouthpiece which is open at both ends, for connection to the pipe mouth; a stopper for shutting off one end of the mouthpiece, which stopper has an axially projecting part which is intended for insertion into the mouthpiece; and a lock which is placed by a key in a locked and a released position, in the locked position the stopper on the mouthpiece being securely connected to the mouthpiece and the key being freely movable into and out of the lock, and in the released position the stopper being freely movable relative to the mouthpiece and the key being securely connected to the lock, the lock being integral with the stopper of which the projecting part comprises: a first locking element whose movement is coupled to that of the key and in the locked position of the lock, when the stopper is on the corresponding mouthpiece, is in engagement with the mouthpiece for mechanical locking of the stopper on the mouthpiece; and a second locking element which in the released position of the lock blocks the first locking element when the stopper is off the mouthpiece, and releases the first locking element when the stopper is on the corresponding mouthpiece, the mouthpiece and the stopper being provided with interacting control means for the operation of the second locking element.
Such an assembly of mouthpiece, stopper or cap and lock is known from French patent specification No. 1194598, and is e.g. used to prevent unauthorised or undesirable access to tanks of liquids such as fuel, foods etc. This securing can be desired for different reasons.
A first reason can be the risk of theft of a part of the liquid. In particular, if the pipe mouth is in the open air on freely accessible ground, and continuous guarding or constant supervision is impossible or impractical, theft can occur, with a greater likelihood as the unit price-of the liquid increases. One example is the theft of motor fuel from a vehicle.
Secondly, the chance of an act of sabotage or vandalism can lead to security measures designed to prevent the undesirable addition of substances which make the liquid unusable, such as the addition to liquid fuels of substances which impede the pumping of the fuel or prevent its combustion, or which in the process are converted into undesirable residues of solid material. If the tank contains a liquid for human or animal consumption, it must be possible to prevent the addition of undesirable or dangerous substances such as a dye or a poison to it.
Thirdly, limits can be set for access to a tank, from the point of view of safety, e.g. in order to prevent mistakes.
The access security measures taken for the above-mentioned reasons generally involve the manufacture of the mouthpiece and the stopper from a strong material which has great resistance to mechanical and chemical Influences, and the use of a lock which can be opened only with a special key.
In the-prior art, if the stopper is not on the mouthpiece, the first locking element, and the lock coupled to it, is blocked by the second locking element, so it is impossible to remove the key from the lock. This locking is not removed until the stopper is in the mouthpiece, in which case the control means can act on the second locking element and thus remove the blocking of the first locking element. A key therefore principally cannot be used for the lock of more than one stopper at a time. This is advantageous if the key and the cylinder belonging to it are protected, so that a loose key principally ensures that the corresponding stopper is securely connected to the corresponding mouthpiece. In this way interchanging stopper is also impossible. Depending on the lock system chosen, working with authorised users means that a secure system of access to liquid tanks can be achieved.
However, the blocking of the first locking element by the second locking element in the prior art is mechanical and can be released quite simply with a simple tool when the stopper is not on the mouthpiece. Thereby the key can be released and the risk of fraud is relatively high, since the key can be moved into the position for locking without the stopper being on the mouthpiece, with the result that key and lock can be separated from each other without the pipe mouth actually having been secured. This makes it difficult to check that the security regulations on it have been carried out correctly.
Furthermore, in particular in the situation outlined above, it is possible to interchange stoppers of corresponding shape coming from different mouthpieces, even if only one key is available for several stoppers. This again can be undesirable in certain circumstances.